A fluorescent lamp is widely used as a light source of an interior lamp, a street lamp, various types of home electric appliances, and so on. In such a fluorescent lamp, a decompressed glass tube is used. Generally, the decompressed glass tube comprises a glass tube having an inner wall coated with a fluorescent material. In the glass tube, a rare gas, such as a neon gas and an argon gas, and a small amount of mercury are confined. In the glass tube, discharge electrodes are also disposed. By applying an electric voltage between the discharge electrodes, discharge occurs to excite or stimulate mercury so that ultraviolet ray having a wavelength of 254 nm is emitted. When the ultraviolet ray is irradiated to the fluorescent material, the fluorescent material is excited to emit visible light. Thus, the lamp is realized.
The fluorescent lamp is classified into a hot cathode fluorescent lamp for emitting thermal electrons to excite mercury and a cold cathode fluorescent lamp for emitting electrons by applying an electric voltage between electrodes, thereby exciting mercury. Both of the hot cathode fluorescent lamp and the cold cathode fluorescent lamp perform light emission when the fluorescent material is excited by the ultraviolet ray of 254 nm emitted by the excited mercury and emits the visible light.
Generally, a glass tube is used as a discharge tube. The fluorescent material is generally classified into a long-wavelength excitation type (red) material, a medium-wavelength excitation type (green) material, and a short-wavelength excitation type (blue) material. For example, a white lamp emits white light by mixing red, green, and blue materials in a desired ratio. The fluorescent material generates visible light when a dopant such as europium present on its surface is excited.
Generally, the fluorescent material has a particle size not smaller than 2 μm. The fluorescent material is applied onto the inner wall of the lamp so that the ultraviolet ray emitted inside the lamp is irradiated to the fluorescent material to cause the visible light to be emitted outside the lamp. For this purpose, the fluorescent material is formed as a layer having a thickness of about 10 μm.
Japanese Unexamined Patent Application Publication (JP-A) No. 2003-027051 discloses the technique using a composite fluorescent material comprising a fluorescent material having a small particle size and adhered to an inorganic compound having a large particle size.
For the fluorescent lamp known as a low-power-consumption lamp, a yet higher efficiency is pursued and lower power consumption is required in view of energy consumption. In particular, a cold cathode lamp used as a backlight of a liquid crystal display of a home electric appliance, such as a personal computer and a television, accounts for a high percentage of power consumption and, in case of a large liquid crystal television of 32 inch or more, the percentage is as high as about 40% of power consumption thereof. Therefore, the cold cathode lamp is required to have yet lower power consumption for use in a home electric appliance of low power consumption.